The present invention relates to a method of controlling a thermal head which is installed in a thermal printer, a facsimile apparatus, and other thermal recording apparatus.
Many printers and other recording apparatus, use a thermal print head to reduce low noise and to bring about other desirable features of a non-impact system. A thermal head generally comprises a great number of heating elements which are arranged in an array at a predetermined pitch. Energy is selectively applied to the heating elements in response to a print signal to heat a heat-sensitive sheet or an ink sheet, whereby dots are printed out on the sheet to record characters and other images.
Some modern thermal printing apparatus are provided with an implementation for the correction of energy to be applied in order to enhance quality printing. This stems from the fact that when a certain heating element is to be energized, applying the same energy to the heating element neglecting the history of the preceding line, that of surrounding heating elements, and other heat accumulation conditions of surrounding dots results in irregular print density. The energy or heat correction may be accomplished by setting up drive conditions on the basis of data on surrounding dots and by use of a line buffer, as disclosed in Japanese Patent Laid-Open Publication No. 58-146176 or the like.
Data to be printed out are generally classified into two kinds, i.e., graphic data such as pictures, and text data such as characters which are stored in a character generator. It is a common practice to selectively energize heating elements of a thermal head based on graphic and text data which are fed from a host machine to the thermal head. Hence, it often occurs that print data from a host machine include both of graphic and text data at a time. Even when graphic and text data coexist with each other as stated, the control system described in Japanese Patent Laid-Open Publication No. 58-146176 controls all print data under the same condition. Such a control system may be effective so long as the print data are graphic data only. However, when it comes to graphic data which are accompanied by text data, the control system disclosed which is elaborated to print out graphic data by use of dots is apt to cause text data, or line data, read out of a character generator to appear thin and, thereby, degrades the printout quality.
It has been customary to drive heating elements of a thermal head by dividing them into two groups, one corresponding to odd dots and the other to even dots. Nevertheless, a problem arises that the preceding dots cause the following dots to render the areas of the latter irregular.